1. Field of the Invention
This invention relates to the production of thermoplastic resin compositions having organic fillers blended therein. More particularly, it relates to a process and an apparatus for producing resin compositions comprising a thermoplastic resin such as polypropylene and a fibrous organic filler blended therewith (which resin compositions will hereinafter be referred to as organic filler-blended resin compositions) suitable for molding interior materials for motorcars, building materials, appliances or other industrial materials, according to extrusion molding process.
2. Description of the Prior Art
As to organic filler-containing resin compositions, Japanese patent application laid-open Nos. Sho 56-34737 and Sho 57-115437 disclose in details a polyolefin resin composition consisting of a blend composed mainly of a polyolefin resin and vegetable fibers. The vegetable fibers used for the composition include finely-ground pulp, finely-ground waste paper such as newspaper, magazine, corrugated board, etc., finely-ground non-woven fabric, cotton cloth, etc., finely-ground wood chips, etc. The applications also discloses that as to the fiber length thereof, especially long fibers are undesirable and in order that the fibers are uniformly dispersed in polyolefin resin when blended with the resin, it is desirable that fine grinding be sufficiently carried out and interfibrous entanglements be freed; thus practically it is desirable that the fiber length be 300.mu. or less and the fiber diameter be 30.mu. or less. Further, Examples thereof illustrate a process for preparing pellets of vegetable filler-blended polyolefin resin, wherein the vegetable fibers and polyolefin resin and additives such as plasticizer, rosin, petroleum resin or inorganic filler, etc. are mixed in definite amounts and the mixture is kneaded on heating by a Banbury mixer, a kneader, or the like, followed by being introduced into an extruder when the resin in the mixture melts and begins to be impregnated into the vegetable fibers.
In this process, in order to improve the dispersibility of the mixture at the time of kneading, it is necessary to finely ground the vegetable fibers in advance, as described above. In order to effect this, it is necessary to roughly grind organic materials such as waste paper to a certain size once and then feeding them into a specified finely-grinding machine of e.g. turbo type to thereby finely grind them till a state is attained where the bonds and entanglements between the finely-divided fibers are freed. However, it is so difficult to handle thus finely-divided fibers that particular cares are required.
Namely, in view of the specific features of powder, the smaller the particle size in a storage tank or a feeding apparatus where how large its volume is, is important, the greater the porosity (a ratio of the total volume of interparticle spaces to the total volume of powder plus the volume of the interparticle spaces), and the lower the bulk density. This has been considered to occur because of the fact that in the case of finely-divided particles, interparticle adhestion increases, resulting in much larger clearances than the size of the particles at various parts within the powder layer. Further, in view of the adhesion, there is a tendency that the smaller the particle size, the larger the angle of repose. This phenomenon is one of the factors for the liability of occurrence of clogging of finely-divided particles at a discharge port and incapability of discharging the particles when a storage apparatus is provided during the process of transporting the particles, that is, a so-called bridge formation. Finely-ground vegetable fibers such as those disclosed in the above references are particularly strong in this tendency. Further, when the fibers and a thermoplastic resin in the form of pellets are mixed together and fed, notable differences of the particle sizes and the shapes of the both result in differences in bulk density, angle of repose, fluidity, etc. of the resulting material; thus the finely-ground fibers may separate from pellets at the feeding and discharging stage, that is, the so-called segregation phenomenon may occur.
These various phenomena which occur due to such factors originating from the finely-ground fibers, make it difficult or impossible to feed and discharge the material; thus the phenomena have a great influence upon the performance of the apparatus. Therefore, when an apparatus is designed, it is necessary to use a particular measures for preventing the above bridge formation and segregation phenomenon. Further, industrially complicated, various prior steps such as finely-grinding step, metering and transporting step, etc. for the vegetable fibers are also required. Thus such a manner has disadvantages that the initial cost increases and manpower is also required.
As another prior art directed to production of organic filler-containing resin composition, Japanese patent publication Nos. Sho 56-9576/1981 or Sho 57-43575/1982 discloses a process wherein vegetable fibers are not finely but roughly ground to a certain size and mixed, as they are, with a thermoplastic resin to produce a composite resin composition.
According to the prior art, a thermoplastic resin such as polyethylene, polypropylene, etc. is mixed with roughly-ground pieces obtained by chopping waste paper e.g. newspaper, corrugated board, compressed board paper, etc. to a size of about 5 mm.times.5 mm by means of a cutter or a turbo mill, and the mixture is introduced into a mixer heated in advance (practically a Henschel type mixer has been employed), followed by kneading through a high rate shear flow by means of agitation holes revolving in the mixer at a high rate. As the kneading advances, the temperature of the above mixed materials in the mixer rises due to the heat generated by friction and shear, the moisture of the waste paper vaporizes and the waste paper is dried. At the same time, the molten thermoplastic resin is impregnated into the waste paper, and beating of the paper is carried out in the resin-impregnated state. When the beating has advanced sufficiently, the revolving velocity of the mixer is reduced and the mixture is subjected to nucleus-creation at the initial period of granulation, followed by transferring it to a mixer cooled with water to about 20.degree. C., and granulating under cooling at a low velocity revolution to obtain a thermoplastic resin composition having finely-divided vegetable fibers blended therein.
However, polyethylene and polypropylene resins have a nature of being degradated by oxidation when they are contacted with air in a heated, molten state. Thus, according to the above process for producing the resin compositions utilizing a heating and kneading mixer of high velocity shear flow type, the resin flows together with air flow caused in the mixer by its high velocity revolution during which the resin is brought into a molten state by generated heat, so that the resin temperature reaches 220.degree. C. or higher; thus the resin is affected by oxygen in air and an oxidation reaction occurs within the resin itself which necessarily promotes degradation thereof.
According to the above process, nucleus-creation and granulation are carried out while the molten thermoplastic resins are impregnated into vegetable fibers, by the shear flow of the miser revolving at a high velocity; hence a flow variation within the mixer is liable to present a quality problem that compositional unhomogeneity of the mixture, the ununiformity of particle diameter, etc. occur partially in the mixture. Further, the extent of interparticle bonds and entanglements is so large that when roughly-ground pieces of waste paper, wood chips or the like are used, there is a drawback that fiber disentanglement by agitation flow of the mixer is not sufficiently carried out. Furthermore an operational problem is raised: since treatment is carried out in a mixer having a specified volume, the operation is batchwise and the treating time is long (46-47 minutes are required in the Examples); productivity does not increase and power saving is difficult.
Thus, in order to overcome the drawbacks of the above prior art, the present inventors pay attention to a process wherein a fiber-rich organic material in the form of roughly-ground small pieces and a thermoplastic resin are introduced together and directly into an extruder to subject the composition to extrusion molding, without a melt-kneading step in air atmosphere, as disclosed in Examples of the above Japanese patent publication No. Sho 56-9576/1981. However, in the case of monoaxial or biaxial screw type extruders which have so far been generally used for extrusion molding of thermoplastic resins, even if they have a sufficient kneading function for thermoplastic resins, it has been very difficult to uniformly disperse the above organic materials into the molten thermoplastic resin in the extruders to make it impossible to obtain a homogeneous composition. The cause has been considered to be in the following fact: in order to obtain a homogeneous, organic filler-blended resin composition, it is necessary for extruders to have a function of finely disentangling the bonds and entanglements of the fibers in the roughly-ground materials (which will hereinafter be abbreviated to fiber disentanglement), in the state where thermoplastic resin which has melted during the kneading process in the extruders is impregnated into roughly-ground organic fillers, but the above function is insufficient in the case of the screw form of conventional monoaxial or biaxial screw type extruders.
In order to solve the above-mentioned various problems, the present inventors have conducted extensive research on a new and functionally superior process for producing organic filler-blended resin composition and an extruder used therefor, unlike conventional process for producing the compositions by means of conventional types of extrusion molding or the above heating and kneading mixer, and as a result achieved the present invention.